Prevalence, determinants, intervention strategies and current gaps in addressing childhood malnutrition in Vietnam: a systematic review

Background Childhood malnutrition in all forms is a major public health issue worldwide. This review systematically examined the prevalence and determinants and identify the potential interventions and current gap in addressing malnutrition including undernutrition, overnutrition and micronutrient deficiencies (MNDs) in Vietnamese children aged 0–18 years old. Methods Embase, Scopus, PubMed, and Web of Science were systematically searched through June 2022 to identify relevant articles published within the past 25 years. Study selection and data extraction were performed by one reviewer and checked for accuracy by the other two reviewers in accordance with PRISMA guideline. Risk of publication bias was assessed using American Dietetic Association Quality Criteria Checklist. Results Seventy-two studies that met the inclusion criteria were included. Undernutrition has decreased over time but still 22.4%, 5.2% and 12.2% of children under 5 were stunted, wasted and underweight, respectively. Anaemia, iron, zinc, and vitamin D deficiencies were the more common forms of MNDs, the prevalence varied by age, region, and socioeconomic group. Population-based surveys reported that 11% and 48% of children aged 0–11 years old were iron and vitamin D deficient, respectively. Zinc deficiency affected almost one-quarter of the children and adolescents. Retinol deficiency was of less concern (< 20%). However, more evidence on MNDs prevalence is needed. Overweight and obesity is now on the rise, affecting one-third of school-aged children. The key determinants of undernutrition included living in rural areas, children with low birth weight, and poor socio-economic status, whereas living in urban and affluent areas, having an inactive lifestyle and being a boy were associated with increased risk of overweight and obesity. Nutrition specific intervention studies including supplementation and food fortification consistently showed improvements in anthropometric indices and micronutrient biomarkers. National nutrition-sensitive programmes also provided nutritional benefits for children’s growth and eating behaviours, but there is a lack of data on childhood obesity. Conclusion This finding highlights the need for effective double duty actions to simultaneously address different forms of childhood malnutrition in Vietnam. However, evidence on the potential intervention strategies, especially on MNDs and overnutrition are still limited to inform policy decision, thus future research is warranted. Supplementary Information The online version contains supplementary material available at 10.1186/s12889-024-18419-8.


Introduction
Vietnam is a country in Southeast Asia (SEA), with an estimated population of 96 million.Since 1986, the government has introduced economic reform, aiming to boost the country's economic development [1].Vietnam now has become one of the fastest growing gross domestic products worldwide [2], which resulted in substantial decrease in both poverty and undernutrition in the population.However, inequalities have arisen in social, health, and wealth, whilst overnutrition has become an increasing burden [3].Malnutrition can lead to serious health consequences and increases the risk of childhood mortality and morbidity.Undernutrition affects every system in the body, which results in altered immune function, increased susceptibility to illnesses and the complications include increased respiratory tract infections, impaired cardiac function, and reduced long-term cognitive development [4].The health consequences that arise from MNDs vary widely depending on the types and magnitudes of deficiency of such micronutrients, but can include xerophthalmia (dry eyes, night blindness), anaemia (fatigue and heart palpitations), and goitre (swelling of the neck).Overnutrition is associated with increased risk of diet-related non-communicable diseases such as cardiovascular disease, cancers, strokes, type 2 diabetes, and polycystic ovarian syndrome [5].Thi et al. 2015 proposed that the nutrition transition in Vietnam began in the late 1990's [6] due to rapid economic development, urbanisation, globalisation via free trade and investment, alongside changes in food system, promoted consumption of energy-dense, nutrient-poor foods and sedentary behaviours among children [7].This shift has led to the development of the double or triple burden of malnutrition, with overweight and obesity becoming more prevalent, whilst undernutrition persists [8,9].The latest General Nutrition Survey in 2019 reported that 19.6% and 16.8% of children aged < 5 and 5-19 years old were stunted, respectively, whereas 19% of children aged 5-19 years old were overweight [10].Determinants found to be associated with child malnutrition can be classified as distal (socioeconomic), intermediate (environmental or maternal) or proximal factors (individual) [11].Distal factors include the region of living, ethnicity, mother's education and occupation, and household income.
Intermediate factors were split into two types: environment such as household size, structure of house, latrine type, and water source, and maternal variables such as mother's age at birth, mother's BMI, and number of children.Proximal factors include quality of breastfeeding (duration and initiation time), birth weight, and health status.
Tackling malnutrition is one of the United Nations Sustainable Development Goals (SDGs), specifically 'Goal 2 -Zero Hunger' , needing joint efforts from multiple sectors and disciplines.This requires achieving food security, improving diet and nutrition, and promoting sustainable agriculture and consumption.In general, there are two different types of nutrition interventions: nutrition specific and nutrition sensitive [12].Nutrition specific interventions aim to address any immediate determinants of malnutrition, and these include supplementation and food fortification, which mostly focus on iron, zinc, folic acid or multiple micronutrients, as well as supporting exclusive breastfeeding, and diet diversification [12].In addition, nutrition sensitive interventions aim to address the underlying rootcauses of malnutrition, such as improving food security through agriculture support, nutritional education, safe drinking-water, sanitation, hygiene and improving healthcare access [13].
Given the severity of the malnutrition situation, the key drivers in Vietnamese children have rarely been extensively and systematically reviewed, leading to an evidence gap on the extent and burden of malnutrition and its potential strategies.Therefore, this study aimed to systematically review the current evidence on the prevalence, determinants and intervention strategies of malnutrition including undernutrition (stunting, wasting and underweight), over-nutrition (overweight and obesity) and micronutrient status, particularly that of anaemia, iron, zinc, vitamin A, D, and folate, in Vietnamese children aged 0-18 years old.

Methodology
This systematic review was conducted by three independent reviewers in accordance with the preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines.

Inclusion and exclusion criteria
PI(E)CO (Participant, Intervention/ Exposure, Comparison, and Outcome) framework [14] was used to guide the development of the inclusion and exclusion criteria of this systematic review, and the details are shown in Table 1.All observational studies (cross-sectional and longitudinal) and human experimental studies (randomised, or non-randomised, and controlled, or noncontrolled trials) reporting the prevalence, determinants, and interventions (including both nutrition sensitive and specific programmes) of malnutrition in Vietnamese children aged 0-18 years were included.
Malnutrition indicators included undernutrition which was defined according to the World Health Organisation (WHO) guideline such as stunting (height-for-age Z-score (HAZ) or length -for-age Z-score (LAZ)), wasting (weight-for-height Z-score (WHZ)), thinness (body mass index-for-age Z-score (BAZ)), underweight (weight-forage Z-score (WAZ) or weight-for-length Z-score (WLZ)), and mid upper arm circumference (MUAC)) [4,15].Indicators for overnutrition included overweight and obesity which defined by both WHO and International Obesity Task Force (IOTF), body mass index (BMI), and waist and hip circumferences [16].MNDs particularly focusing on anaemia, iron, zinc, and vitamins A, D, and B9 (folate or folic acid).Only studies that assessed MNDs using biomarkers were included, whereas those that assessed using dietary intake were excluded.
Only studies with access to full text, reported in English and within the past 25 years were included.This time frame has been selected based on Thi et al. 2015 who reported that the nutrition transition in Vietnam began in the late 1990's to capture the emergence of the double or triple burden of malnutrition [6].Studies identified from the search were checked and confirmed with local experts at the National Institute of Nutrition, Vietnam to ensure all or at least most of the relevant studies in this region have been captured.Communication, case reports, letters, editorial matters, conference abstracts and reviews were not included.

Search strategy
The literature search was performed on 29 th June 2022 using four different databases including Embase, Scopus, PubMed, and Web of Science.Details of the search strategies for each database are reported in Table S1.

Study selection
Studies were exported from each database into End-Note (Endnote X7.7.1, Thomson Reuters 2016) and were screened by three independent reviewers using the inclusion and exclusion criteria.When records were identified from all four databases, the duplicates were removed.During the first stage of screening, all studies were filtered by title and abstract, and those which did not meet the inclusion and exclusion criteria were excluded.The full texts of the remaining records were accessed and screened using the same eligibility criteria.The final decision regarding the eligibility of articles was made by agreement between all reviewers.Disagreement between reviewers were resolved by discussion and by other reviewers when necessary.

Data extraction
Where available, the following information was extracted: author and year of publication, study design and duration, participants' characteristics including inclusion criteria, age and sample size, outcome measures (anthropometric parameters and micronutrients related blood biomarkers), intervention approaches (where appropriate), and key findings.In the case of missing data or unclear pieces of information, it was considered that the Studies that do not assess malnutrition indicators as the primary outcomes or focused on the consequences of malnutrition authors did not report such variables.Any disagreement between the three reviewers was resolved by discussion to reach a consensus, and by involving other authors when necessary.

Data synthesis
The quantitative data from the studies included in this review were analysed.Data were extracted and tabulated manually into a pre-prepared excel spreadsheet based on three different topics of interest: the prevalence, determinants and the interventions that primarily aimed to address childhood malnutrition in Vietnam.Determinants included those factors found to be negatively or positively associated with different forms of malnutrition; the effect size of such associations (e.g., odds ratios (OR) or β-coefficient) were extracted and reported in the tables, and if it was not shown, the p-value was reported.With respect to interventions, the studies were further stratified into nutrition sensitive or nutrition specific interventions [17].Definition of nutrition specific and sensitive interventions were as mentioned in the introduction section.The details of the interventions including the duration, sample characteristics, intervention approaches, control group (where applicable), and the key findings assessed by the post-intervention changes in child anthropometric indices, or micronutrients biomarkers were reported.

Quality criteria checklist for risk of bias
The risk of bias was assessed for each study using a Quality Criteria Checklist from the Academy of Nutrition and Dietetics [18].This includes questions on [1] research question clarity, [2] selection bias, [3] study group comparability, (4) withdrawal description, (5) blinding, (6) interventions, (7) study procedure description, (8) appropriate statistical analysis, (9) supported conclusions and limitations, and (10) sponsorship or funding bias.Depending on the answers to these questions, each study was assigned a negative, neutral, or positive rating.To be rated positive, five of the questions must be answered yes, including questions 2, 3, 6 and 7, and one other yes.The risk of bias was assessed by one reviewer and crosschecked by the other two reviewers.The results were summarised in a horizontal stacked bar graph, Fig. 2, showing the percentage of studies which had a low, medium, or high risk of bias for each question.

Results
As shown in Fig. 1, the initial database search strategy identified a total of 2,326 records.This included 722 from Embase, 620 from PubMed, 571 from Scopus and 413 from Web of Science.Two additional records were identified from the reference list of records from the original search.After removing the duplicates, 411 records were screened by title and abstract to check for eligibility based on the inclusion and exclusion criteria.Of those, 242 articles were removed, and 169 full-text articles were assessed for eligibility.A final total of 72 articles which met the inclusion criteria were then included in this review.

Risk of bias assessment
The results for the risk of bias assessment are reported in Fig. 2. 79% of the records were classified as low risk of bias and 21% as unclear risk of bias.The common reasons for lower quality rating were typically the lack of blinding (69% of the records), lack of a withdrawal description (47%) and lack of randomisation or group comparability (17%).All studies stated that the research question, procedure, and the outcome measures have been sufficiently described.

General characteristics of the studies included
Of the 72 articles included in this review, there were  3).

Prevalence of different forms of malnutrition
The average age of participants was 5.3 years, and most of the participants were preschool or primary school-aged children (80%).Over 675,562 children were assessed over a period of 25 years, from 1988 to 2022.Table 3 shows the findings on the prevalence of malnutrition in Vietnamese children aged 2 months to 18 years old.Overall, the prevalence of overweight and obesity showed an increasing trend, especially in recent years.In Hanoi, the prevalence of overweight and obesity increased from 15.5% in 2013 to 21.2% in 2016 [19].The prevalence of overweight and obesity in children aged 4-5 years old increased from 21.4% in 2002 to 36.8% in 2005 in Ho Chi Minh City [20].A recent large-scale cross-sectional study in Ho Chi Minh City reported that overweight and obesity were more prevalent in primary school children, affecting 51.2% of them, compared to secondary school children (35.6%) and high school children (19.2%) [21].Lower prevalence was observed in younger children aged 1-2 years old (10.7%) [22].The prevalence of undernutrition has gradually decreased since the 1990s.A large-scale longitudinal study reported that from 1990 to 2005, the prevalence of stunting in children aged under 5 years old has reduced from 56.5% to 30.7%, wasting from 9.4% to 7.7%, and underweight from 40.0% to 26.6% [23].In 2014-2015, it is reported that 2.4%, 3.8%, and 7.9% of the primary, secondary, and high school aged children were stunted, and 2.2%, 4.6%, and 6.0% were wasted, respectively [21,22].Other studies also reported that the prevalence of stunting was higher in children aged under 6 years old (16-36%) [21,22], compared to children aged 6 years old and above (14%) [44].There were more underweight children in younger age groups, affecting 50% of children aged 1-6 years old [25] compared to 22% in children aged 7-9 years old [44].
The most commonly reported MNDs were iron, zinc, and vitamin A. The prevalence of each MND differed between studies, depending on the micronutrient      Similar finding was found SEANUTs I which reported that vitamin D deficiency was found in 48% of the children aged 0-11 years old [37], whereas a smaller-scale study reported that vitamin D deficiency was affecting 29% of children of same age group [38].Only one study reported the prevalence of calcium deficiency, with 97% of 0-5 years old being mildly calcium deficient [27].
Males were more likely to be stunted/underweight [23], overweight/obese [39,40,64], and anaemic [32] than females (proximal factors) (Table 5).Moreover, the risk of being overweight or obese was 1.39 times higher in physically inactive children [64], and 2.5 times higher in those living in urban areas (p < 0.05) [32].Maternal nutrition was also associated with both child undernutrition and overnutrition (intermediate factors).Other determinants of being overweight include living in a wealthy household [40], inactive mode of transport to school [8], having overweight parents [63], and low food availability at home in girls only [63].Few studies reported the determinants of MNDs, nonetheless iron deficiency was found to be associated with malaria in ethnic minorities [65]; selenium deficiency and being underweight in adolescent girls [41]; and being stunted and underweight in primary school children from the rural areas [67] (proximal factors).

Interventions and its impacts on addressing different forms of malnutrition
Twenty one articles reported intervention studies addressing different forms of malnutrition, mainly focusing on undernutrition and MNDs (Table 6).No intervention study was found addressing overnutrition.These include 4 nutrition sensitive and 17 nutrition specific programmes.Approximately 8,087 children participated in the intervention studies from 1996 to 2015, in addition to a few large-scale studies evaluating the impact of the national government programme on children's nutritional status.The duration of these studies ranged from 30 days to 2 years, and all studies were randomised control trials (RCT) except for one non-random pragmatic trial, which was carried out without a control/ placebo treatment [68].The interventions mainly targeted younger aged children ranged from 0-8 years old.
Most of the intervention studies (93%) demonstrated the effectiveness of the intervention in improving the children's nutritional status, either through improved behaviours (e.g., optimal breastfeeding, exclusive breastfeeding for longer duration, better weaning practices, children consuming more food and more hygienic practices) in the nutrition sensitive interventions, or improved anthropometric indices and micronutrient biomarkers in the nutrition specific interventions (e.g., micronutrients supplementation and fortification).With respect to nutrition sensitive interventions, an evaluation study reported the success of the Government's National Plan of Action for Nutrition (NPAN) and the National Targeted Programme on Hunger Eradication and Poverty Reduction [3] on improving child's nutritional status through a variety of poverty reduction initiatives.Another three nutrition sensitive intervention studies using growth monitoring, positive deviance, and positive feeding practices approaches also reported significant reduction in the prevalence of severely malnourished [69], undernourished [71], and stunted children [70] through improved eating behaviours and feeding practices.With respect to nutrition specific interventions, milk fortification improved height of infants aged 18-36 months by 1 cm on average (p < 0.01), weight by 0.43 kg (p < 0.01) and BMI-Z score by 0.015 (p < 0.05) after 5 months [86].Similarly, children gained significantly more weight and height compared to the control group, after receiving fortified biscuits and milk which provided an extra 300 kcal/day [73].Children of mothers who received multiple micronutrients supplementation during pregnancy were significantly taller than those receiving just iron and folic acid [77], however such effects were not found in Hanieh et al. 20I4 [74].Infants were at a lower risk of anaemia if their mothers had taken the iron and folic acid supplement daily (OR = 0.31; 95% CI = 0.22-0.43),twice weekly (0.38; 95% CI = 0.26-0.54),or the multiple micronutrient (0.33; 95% CI = 0.23-0.48)during pregnancy compared to the placebo group [83].Combined iron and zinc supplementations significantly improved the levels of haemoglobin (Hb), ferritin, and zinc (p < 0.0001) in children aged 4-7 months old after six months of intervention [72].Similarly, prevalence of iron deficiency and anaemia was reduced in infants and children after receiving fortified foods for 5-6 months compared to the placebo group [78,81,82].Supplementing children with 10mg of zinc daily resulted in weight gain by 0.5 kg on average (+ 0.1 kg, p < 0.001) and height by 1.5cm (+ 0.2 cm, p < 0.001) [80].

Table 4 Findings on the determinants associated with different forms of malnutrition in
Only one uncontrolled trial investigated the effects of vitamin D and calcium supplementation [87], and the authors reported improved vitamin D concentrations (p < 0.001) and bone mineral density (BMD).Another study reported that after one month of intervention, both synthetic β-carotene supplementation (a precursor to vitamin A) and Momordica coincidences (gac) fruit showed significantly higher levels of β-carotene than in control group (p < 0.0001), but plasma retinol concentrations were significantly higher in children receiving gac fruit than those who received synthetic β-carotene (p < 0.01) [85].

Prevalence of different forms of malnutrition in Vietnamese children
Our findings revealed that undernutrition (e.g., stunting) and MNDs (e.g., anaemia, iron, zinc and vitamin A deficiencies) were more prevalent in Vietnamese children aged < 5 years old compared to older children, whereas prevalence of overweight and obesity were rising in older children especially among primary school-aged children.Childhood undernutrition has gradually reduced over time in Vietnam [23].This decrease in undernutrition aligns with the country's Hunger Eradication and Poverty Reduction programme (HEPR) launched since 1992, which resulted in improved infrastructure in poor communities, greater financial support for ethnic poor families, and growth of the agriculture and aquaculture industries.In 1995, the NPAN indicated the government's serious commitment to reducing malnutrition in the country [3].These national programmes have shown positive outcomes on poverty reduction, subsequently resulting in improvement in child's nutritional status and reduction in child undernutrition and MNDs [3].
This finding was in line with the data reported by the UNICEF/WHO/World Bank Group [88] (Fig. 4), of a steady decline in wasting, stunting and underweight from 1998 to 2019, and a slow increase in overweight in children aged < 5 years old.Vietnam has made substantial progress towards achieving the targets set for the SDGs, which is to reduce the prevalence of stunting and wasting in children aged < 5 years old to below 20% and 5%, respectively [89].However, ethnic minorities, especially those residing in poor and remote areas, are still being left behind, thus limiting the progress on national reductions [89].This emphasises the urgent needs to focus prevention implementation on the most vulnerable populations, with strategies to address socioeconomic inequalities.On the other hand, Vietnam has failed to prevent the increase of childhood overweight and obesity.It is worth noting that for the first time in 2017, the prevalence of overweight in children aged < 5 years old overtook the prevalence of wasting in Vietnam.This has also been reflected in the increasing number of studies investigating childhood overweight and obesity since 2015-2022.
Compared to neighbouring SEA countries, prevalence of overweight children aged < 5 years old (3.5%) in Vietnam was lower than in Indonesia (11.5%) and Thailand (10.9%) in 2015 but higher than in Myanmar and Cambodia (< 2%) [90].The prevalence of underweight in children aged 0.5-11 years old was lower in Vietnam (10.9% urban; 20.1% rural) compared to Indonesia (25.1% urban; 39.2% rural) but higher than Thailand (6.4% urban; 10.2% rural).Prevalence of anaemia was higher in Vietnam (22.5% urban; 27% rural) than Thailand (9.0% urban; 18.4% rural) and Indonesia (17.6% urban; 18.5% rural).Thus, the nutrition situation in Vietnam is not extreme for the region, with differences likely due to different economic and political circumstances, as well as different staple diets in each country.

Determinants of childhood overweight and obesity
According to the 'nutrition transition theory' proposed by Popkin 1994 [91], rapid urbanisation and economic development, have contributed to the nutrition transition in Vietnam, from traditional diets rich in fresh fruits, vegetables and rice, to diets that are high in processed foods, refined grains, meat, dairy products, saturated fats, and sugars and are low in fibre [92,93].The price of food in Vietnam, especially high-sugar and processed foods decreased during the nutrition transition, making them increasingly accessible [94].Unhealthier foods have become increasingly marketed towards children and accessible in outlets such as supermarkets, convenience shops and school cafeterias [7].This has resulted in an increased risk of childhood overweight and obesity, and the development of double or triple burden of malnutrition.
Boys (proximal factors) are more likely to be overweight and obese compared to girls in Vietnam, especially in urban areas.Biological influences on diet (e.g., higher energy intake in boys), and sociocultural influences at individual, parental and familial level [95] are potential contributing factors.Girls are more likely to experience a higher-level of weight-related concerns, including desire to lose weight, compared to boys due to gender-based stereotypes [96].Another reason could be due to male chauvinism, where parents regard boys as more valuable than girls [97], and parents may be more incline to overlook overweight and obesity status in sons compared to daughters [98].In terms of intermediate risk factors, children living with their father only, had increased risk of being overweight and obese, compared to those with both their mother and father [39].A possible explanation for this increased risk could be due to fathers being less likely to limit food access and monitor children's food intake [99].However, due to the low divorce rate in Vietnam, this may only represent 1.15% or even less of the population.
Children living in urban areas (distal factors) were more likely to be overweight and obese compared to rural areas [8].This may be due to increased sedentary lifestyle, inactive transport methods to school (bus, car, motorbike), no recess activity due to fewer open spaces and playgrounds, more processed foods and fast food restaurants, and neighbourhoods that are less safe for walking and outdoor activities, alone [5,19,39,55,56,63], which may have promoted weight gain in children.Although most studies in Vietnam reported that children living in wealthier families had a higher odd of obesity, but a recent study in Ho Chi Minh underscored that children living in lower-income families in urban areas are likely to have a higher odd of obesity than those living in higher-income families [7].This may be attributed to various barriers to healthy eating and lifestyle unique to rural settings such as increased availability of low-cost energy-dense foods, inadequate grocery stores, lack of exercise facilities, and lack of nutritional knowledge [100,101].This trend may be similar in high-income countries.

Determinants of child undernutrition and MNDs
It is well accepted that undernutrition and MNDs mainly results from inadequate intake of energy and nutrients, malabsorption, impaired metabolism, loss of nutrients, or increased nutritional requirements [102].With respect to the distal factors of undernutrition, previous evidence reported that although much effort has been made to control child undernutrition and MNDs in Vietnam, prevalence still remains high, especially in ethnic minorities residing in poor and remote mountainous areas [65,103].Reasons may include food insecurities [104], poor diet quality, low socioeconomic status, low quality of health and nutrition service and high incidence of intestinal parasitic infections [21,105].
With respect to environmental intermediate factors, improved household sanitation can reduce exposure to diarrhoeal diseases [48], reducing the risk of undernutrition and MNDs [106].Household size was found to be positively associated with undernutrition [11,23,51], as children in large households may receive lower quantities of food, resulting in inadequate energy and nutrient consumption.In addition, preschool attendance, with three meals a day included, was found to be negatively associated with underweight and stunting [50].Maternal factors, such as the mother's height, BMI, and nutritional status were found to be positively associated with child growth indicators [107], emphasizing the need for pregnant women as well as women of reproductive age to have a healthy weight, or BMI, to increase the likelihood of producing healthy children.Parental mental disorders were also associated with child underweight and stunting [28,46] which can be explained by the parent's hindered ability to take care of their children and poor feeding practices [28].
With respect to proximal factors, children who were born with LBW were associated with increased risk of stunting, wasting, and underweight [11,28].LBW is a result of poor maternal nutritional status during pregnancy or adolescence, and preterm birth [108], and is commonly associated with reduced immune function and increased vulnerability to illness.Furthermore, our findings also emphasise the importance of optimal breastfeeding and complementary feeding practices in ensuring children receive adequate amount of nutrients for optimal growth, and to protect against infections [109].Other studies reported that anaemia was mainly attributable to inadequate dietary iron intake, although malaria, deficiencies in other micronutrients such as vitamin A and selenium, being underweight, and age are also contributing factors [41,65].It is reported that young boys were at a higher risk of anaemia than girls [8].This aligns with previous evidence that indicated that males are at a higher risk of iron deficiency compared to girls when aged < 10 years old [110].It has been suggested that the hormonal effects on erythropoietin activity for red blood cell production, and higher pre-and postnatal growth rate in males (e.g., differences in body weight or proportion of lean and fat mass may affect iron metabolism) may have increased the susceptibility to anaemia in young boys [33,61,62].Whereas in girls aged > 10 years old specifically, anaemia is likely attributable to regular blood loss through menstrual bleeding which depletes their iron stores [111].Nonetheless, future investigations are needed to explore the effects of malaria and both nutritional and non-nutritional factors on anaemia.
Zinc deficiency varied by age, with a higher prevalence found in children aged 1-6 years old than those aged 6-11 months.Although the underlying reasons are yet to be explored, it is widely accepted that in low-and middle-income countries like Vietnam, the predominant rice-based diet with little animal produce, may result in insufficient dietary zinc intake to meet the high requirement needed for rapid growth at this age [84].Serum zinc biomarker is known to be prone to over-estimation errors due to contamination at field collection and testing stage, which likely contributes to inaccurate assessment.However, the interpretation of the key determinants of MNDs is limited by the scarcity of MNDs data, primarily due to the limited investigation, laboratory facilities and testing capacity.

Intervention and its effectiveness in addressing different forms of malnutrition
The majority of the intervention studies in Vietnam focus on addressing child undernutrition and MNDs in both rural and urban areas.Most studies investigated the impacts of daily multi-micronutrient supplementation (e.g., 13-15 micronutrients) or fortified foods (e.g., milk and biscuits) on children's nutritional status.The dosage effects of supplements were investigated across different intervention studies, specifically in iron (10-60 mg), and folic acid (0.4-1.5 mg), whereas the dosage of zinc supplements given in the interventions were more consistent (10 mg).All supplementation interventions reported improvement in the child's nutritional status in at least one of the outcomes.
Iron fortification has shown a positive effect on Hb, ferritin and body iron levels after 6 months of intervention in anaemic children from rural Vietnam who were not iron deficient [78].This indicates that iron fortification may be effective in improving anaemia and iron status of anaemic children.The Government Decree 09/2016/ND-CP has mandated the fortification of salt with iodine, wheat flour with iron and zinc, and vegetable oil with vitamin A, which has been applied by many other countries, including developed countries.However, the implementation of Decree 09 still faces many difficulties in Vietnam.It is worth noting that two studies consistently reported negative interaction between zinc and iron co-supplementation on serum zinc levels, WAZ, or anaemia [72,84].Zinc supplementation alone was more effective at improving zinc deficiency and WAZ, whereas iron supplementation alone was more effective at reducing anaemia, compared to combined zinc and iron supplementation.The mechanism for this interaction between zinc and iron is likely due to the competitive absorption through a shared, non-specific pathway [112].It is suggested that taking each supplement 30 min apart would be enough to limit their interactions and allow sufficient absorption of each micronutrient.
NPAN is considered one of the largest scaled national nutrition sensitive intervention programmes involving a variety of poverty reduction initiatives, and was found to be the most effective intervention in reducing child malnutrition [3].Other nutrition sensitive intervention programmes included deworming, growth monitoring, nutritional rehabilitation, loan programmes, improved health care and positive deviance inquiry.Deworming showed no impact on Hb, ferritin and body iron status, and authors postulated that this may be due to the 'light' or 'mild' Trichuris infection among the children in this study population and hence showing an absence of effects.Thus, the effectiveness of deworming in improving Hb status in this population is still questionable and future research is warranted, particularly in areas where there are high rate of infection/inflammation [78].Watanabe et al. 2005 reported a decrease in the prevalence of stunting following a nutrition intervention of growth monitoring and nutrition education [70], this intervention involved educating children and parents about locally available nutritious foods.Furthermore, positive deviance approaches and childcare practices which led to improved feeding practices and eating behaviours have resulted in improvement in anthropometric indices [69,71].

Current evidence gaps and future recommendations
Despite the large amount of data collected and analysed, there is limited evidence on the determinants, especially for MNDs and overnutrition.No intervention studies were found aimed at addressing childhood overweight and obesity in Vietnam, suggesting the need for more research to fill in evidence gaps on effective approaches to combat childhood overweight and obesity.Future intervention studies should focus on "double-duty actions" that aim to simultaneously address multiple forms of malnutrition.Particularly for intervention studies, it is essential to include randomisation and blinding procedures and withdrawal description (where applicable), to improve the validity and quality of the study outcomes.
The National Nutrition Strategy (NNS) 2021-2030, with a vision towards 2045 which was ratified by the Vietnamese Prime Minister (NNS Decision No. 02/QD-TTg) has also included indicators on overweight and obesity in children and adolescents, showing the Vietnamese government's commitment in addressing the emerging issue of overweight and obesity [113].Vietnamese government's priorities align with the WHO plan for controlling the global obesity epidemic, which is to promote a whole-society and cross-sectoral approach, to address the obesogenic environment that is negatively influencing children's health and nutrition [114].However, more evidence on effective strategies is needed to better develop and implement such policies and intervention programmes, including introduction of a sugary sweetened beverages (SSB) tax, restricting the marketing and advertising of unhealthy foods and drinks, and front of pack nutrition labelling [113,115].Joint efforts from local government, schools, parents, and food industries will be required to ensure the availability and accessibility of more affordable and healthier options of food products to make them more appealing to consumers.Education programmes should be strengthened to generate public awareness on healthy eating and active lifestyle to promote healthy eating behaviour and lifestyle in children and parents/caregivers.

Strengths and limitations
A key strength of this review is the comprehensive and systematic review of the existing evidence on all forms of childhood malnutrition (undernutrition, overnutrition and MNDs) in Vietnamese children.The findings from this review allow us to identify the shared determinants of different forms of malnutrition and the type of interventions available in this region, which is essential to develop effective double-duty actions to simultaneously address all forms of malnutrition.The use of the standard PRISMA method for a systematic review and a standardised search strategy means that this review can be reproduced and compared with other reviews following the same format and using the same search strategy.One of the limitations of this review is the heterogeneous nature of the study outcomes, limiting the possibility to perform a meta-analysis on the extracted data.Lack of available data prevents reliable conclusions to be drawn, especially regarding MNDs and overnutrition, thus future research is warranted.Only studies reported in English were included, and this may have limited the scope.

Conclusion
Undernutrition is still affecting one quarter of Vietnamese children aged under 5 despite a significant reduction over the last decade.MNDs remain a serious problem, particularly deficiencies in iron, zinc, and vitamin D, whilst childhood overweight and obesity are rapidly rising in school aged children, indicating the existence of the triple burden of malnutrition in this region.This finding highlights the needs for double duty actions, and these actions should be tailored based on regional-level needs, with strategies to address demographic and socioeconomic disparities to simultaneously and effectively address different forms of child malnutrition in Vietnam.Most of the nutrition-specific intervention strategies including supplementation and food fortification demonstrated positive outcomes; either through improvements in anthropometric indices, micronutrient biomarkers, or diet-related behaviours.Large-scale, nationwide nutrition-sensitive intervention programmes that collectively address different environmental influences on children's health and food choices such as food availability, education, health care, and access to water and sanitation are more likely to be effective in promoting optimal child growth in the long term.However, evidence on the key determinants and effective intervention strategies, especially on MNDs and childhood overweight and obesity are still too limited to adequately inform policy decision, thus future research is warranted.

Fig. 1
Fig. 1 PRISMA flow diagram of identification and selection of studies

Table 2
Number of studies reported by age group and types of malnutrition investigated

Table 3
Findings on the prevalence of malnutrition (undernutrition, overnutrition and micronutrient deficiencies) in Vietnamese children aged 0-18 years old

Table 3
(continued) BMI body mass index, BMI-Z body mass index for-age-Z score, HAZ height-for-age-Z score, Hb haemoglobin, IOTF international obesity task force, LAZ length-for-age-Z score, LMAC left mid arm circumference, MUAC mid upper arm circumference, ND no data, WAZ Weight-for-age-Z score, WHO World Health Organization, WHZ weightfor-height-Z score, WLZ weight-for-length-Z score, 25(OH)D 25-hydroxy vitamin D

Table 4 (continued) Authors and year Study duration & design Sample age Sample size Measurements Key findings -risk factors / determinants Risk of bias
[53]h et al. 2021[53]

Table 5
Number of studies reporting shared determinants of different forms of malnutrition in Vietnamese children aged 0-18 years old + Positive association,-Negative association, ± conflicting results.CMD common mental disorderMalnutrition Type Rurality

Table 6
Findings on the interventions and its impacts on addressing different forms of malnutrition in Vietnamese children aged 0-18 years old

Table 6 (
continued) BMD bone mineral density, BMI body mass index, BMI-Z body mass index for-age-Z score, CRP C-reactive protein, HAZ height-for-age-Z score, Hb haemoglobin, IDA iron deficiency anaemia, LAZ length-for-age-Z score, LBW low birth weight, MMN multi-micronutrient, WAZ weight-for-age-Z score, WHZ weight-for-height-Z score, PF plasma ferritin, PTH parathyroid hormone, RCT randomised controlled trial, SF serum ferritin, sTfR serum transferrin receptors